Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Solar cells, also known as photovoltaic (PV) devices, are specifically used to generate electrical power. PV devices are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein the term “resistive load” refers to any power consuming or storing device, equipment or system.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g. crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term “semiconductor” denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms “photoconductor” and “photoconductive material” are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation of selected spectral energies to generate electric charge carriers. Solar cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. Present commercially available amorphous silicon cells have stabilized efficiencies between 4 and 8%. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs.
PV devices typically have the property that when they are connected across a load and are irradiated by light they produce a photogenerated voltage. When irradiated without any external electronic load, a PV device generates its maximum possible voltage, V open-circuit, or VOC. If a PV device is irradiated with its electrical contacts shorted, a maximum short-circuit current, or ISC, is produced. When actually used to generate power, a PV device is connected to a finite resistive load and the power output is given by the current voltage product, I×V. The maximum total power generated by a PV device is inherently incapable of exceeding the product, ISC×VOC. When the load value is optimized for maximum power extraction, the current and voltage have values, Imax and Vmax respectively. A figure of merit for solar cells is the fill factor ff defined as:
                    ff        =                                            I              max                        ⁢                          V              max                                                          I              SC                        ⁢                          V              OC                                                          (        1        )            where ff is always less than 1 since in actual use ISC and VOC are never obtained simultaneously. Nonetheless, as ff approaches 1, the device is more efficient.
When electromagnetic radiation of an appropriate energy is incident upon a semiconductive organic material, for example, an organic molecular crystal (OMC) material, or a polymer, a photon can be absorbed to produce an excited molecular state. This is represented symbolically as S0+hv→S0*. Here S0 and S0* denote ground and excited molecular states, respectively. This energy absorption is associated with the promotion of an electron from a bound state in the valence band, which may be a π-bond, to the conduction band, which may be a π*-bond, or equivalently, the promotion of a hole from the conduction band to the valence band. Inorganic thin-film photoconductors, the generated molecular state is generally believed to be an exciton, i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. The excitons can have an appreciable life-time before geminate recombination, which refers to the process of the original electron and hole recombining with each other as opposed to recombination with holes or electrons from other pairs. To produce a photocurrent the electron-hole pair must become separated. If the charges do not separate, they can recombine in a geminant recombination process, also known as quenching, either radiatively—re-emitting light of a lower than incident light energy-, or non-radiatively—with the production of heat.
Either of these outcomes is undesirable in a photosensitive optoelectronic device. While exciton ionization, or dissociation, is not completely understood, it is generally believed to occur at defects, impurities, contacts, interfaces or other inhomogeneities. Frequently, the ionization occurs in the electric field induced around a crystal defect, denoted, M. This reaction is denoted S0*+M→e−+h+. If the ionization occurs at a random defect in a region of material without an overall electric field, the generated electron-hole pair will likely recombine. To achieve a useful photocurrent, the electron and hole must be collected separately at respective opposing electrodes, which are frequently referred to as contacts. Exciton dissociation occurs either in high electric field regions by field-emission, or at an interface between, e.g., donor-like and acceptor-like materials such as copper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI), by charge transfer. The latter can be viewed as an exothermic chemical reaction, i.e., a reaction in which some energy is released as vibrational energy. This reaction occurs because the energy separation of the dissociated exciton, i.e., the energy difference between the free electron in, e.g., PTCBI, and the free hole in, e.g., CuPc, is smaller that the energy of the exciton prior to dissociation.
Electric fields or inhomogeneities at a contact may cause an exciton to quench rather than dissociate, resulting in no net contribution to the current. Therefore, it is desirable to keep photogenerated excitons away from the contacts. This has the effect of limiting the diffusion of excitons to the region near the junction so that the junction associated electric field has an increased opportunity to separate charge carriers liberated by the dissociation of the excitons near the junction.
Here appreciation should be taken of some of the distinctions between organic photosensitive optoelectronic devices (OPODs) and organic light emitting devices (OLEDs). In an OLED, a bias is applied to a device to produce a flow of holes and electrons into a device. In OLEDs, excitons are generally formed which in time may either recombine radiatively or nonradiatively. In OLEDs, maximum radiative recombination is the desired result. In OPODs maximum exciton generation and dissociation is the desired result. The differing objectives of the devices lead to differing selection of materials and layer thicknesses. OPOD photosensitive materials are chosen for their absorption properties while photoluminescent materials for OLEDs are chosen for their emissive properties.
To produce internally generated electric fields which occupy a substantial volume, the usual method is to juxtapose two layers of material with appropriately selected conductive properties, especially with respect to their distribution of molecular quantum energy states. The interface of these two materials is called a photovoltaic heterojunction. In traditional semiconductor theory, materials for forming PV heterojunctions have been denoted as generally being of either n, or donor, type orp, or acceptor, type. Here n-type denotes that the majority carrier type is the electron. This could be viewed as the material having many electrons in relatively free energy states. The p-type denotes that the majority carrier type is the hole. Such material has many holes in relatively free energy states. The type of the background, i.e., not photogenerated, majority carrier concentration depends primarily on unintentional doping by defects or impurities. The type and concentration of impurities determine the value of the Fermi energy, or level, within the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the HOMO-LUMO gap. The Fermi energy characterizes the statistical occupation of molecular quantum energy states denoted by the value of energy for which the probability of occupation is equal to ½. A Fermi energy near the LUMO energy indicates that electrons are the predominant carrier. A Fermi energy near the HOMO energy indicates that holes are the predominant carrier. Accordingly, the Fermi energy is a primary characterizing property of traditional semiconductors and the prototypical PV heterojunction has traditionally been the p-n interface.
In addition to relative free-carrier concentrations, a significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. As opposed to free carrier concentrations, carrier mobility is determined in large part by intrinsic properties of the organic material such as crystal symmetry and periodicity. Appropriate symmetry and periodicity can produce higher quantum wavefunction overlap of HOMO levels producing higher hole mobility, or similarly, higher overlap of LUMO levels to produce higher electron mobility. Moreover, the donor or acceptor nature of an organic semiconductor, e.g., 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), may be at odds with the higher carrier mobility. For example, while chemistry arguments suggest a donor, or n-type, character for PTCDA, experiments indicate that hole mobilities exceed electron mobilities by several orders of magnitude so that the hole mobility is a critical factor. The result is that device configuration predictions from donor/acceptor criteria may not be borne out by actual device performance. Due to these unique electronic properties of organic materials, rather than designating them as “p-type” or “acceptor-type” and “n-type” or “donor-type”, the nomenclature of “hole-transporting-layer” (HTL) or “electron-transporting-layer” (ETL) is frequently used. In this designation scheme, an ETL will preferentially be electron conducting and an HTL will preferentially be hole transporting. The term “rectifying” denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferably in one direction. Rectification is associated normally with a built-in electric field which occurs at the heterojunction between appropriately selected materials.
The electrodes, or contacts, used in a photosensitive optoelectronic device are an important consideration. In a photosensitive optoelectronic device, it is desirable to allow the maximum amount of ambient electromagnetic radiation from the device exterior to be admitted to the photoconductively active interior region. That is, it is desirable to get the electromagnetic radiation to where it can be converted to electricity by photoconductive absorption. This often dictates that at least one of the electrical contacts should be minimally absorbing and minimally reflecting of the incident electromagnetic radiation. That is, such contact should be substantially transparent. When used herein, the terms “electrode” and “contact” refer only to layers that provide a medium for delivering photogenerated power to an external circuit or providing a bias voltage to the device. That is, an electrode, or contact, provides the interface between the photoconductively active regions of an organic photosensitive optoelectronic device and a wire, lead, trace or other means for transporting the charge carriers to or from the external circuit. The term “charge transfer layer” is used herein to refer to layers similar to but different from electrodes in that a charge transfer layer only delivers charge carriers from one subsection of an optoelectronic device to the adjacent subsection. As used herein, a layer of material or a sequence of several layers of different materials is said to be “transparent” when the layer or layers permit at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through the layer or layers. Similarly, layers which permit some but less that 50% transmission of ambient electromagnetic radiation in relevant wavelengths are said to be “semi-transparent”.
Electrodes or contacts are usually metals or “metal substitutes”. Herein the term “metal” is used to embrace both materials composed of an elementally pure metal, e.g., Mg, and also metal alloys which are materials composed of two or more elementally pure metals, e.g., Mg and Ag together, denoted Mg:Ag. Here, the term “metal substitute” refers to a material that is not a metal within the normal definition, but which has the metal-like properties that are desired in certain appropriate applications. Commonly used metal substitutes for electrodes and charge transfer layers would include doped wide bandgap semiconductors, for example, transparent conducting oxides such as indium tin oxide (ITO), gallium indium tin oxide (GITO), and zinc indium tin oxide (ZITO). In particular, ITO is a highly doped degenerate n+ semiconductor with an optical bandgap of approximately 3.2 eV rendering it transparent to wavelengths greater than approximately 3900 Å. Another suitable metal substitute material is the transparent conductive polymer polyanaline (PANI) and its chemical relatives. Metal substitutes may be further selected from a wide range of non-metallic materials, wherein the term “non-metallic” is meant to embrace a wide range of materials provided that the material is free of metal in its chemically uncombined form. When a metal is present in its chemically uncombined form, either alone or in combination with one or more other metals as an alloy, the metal may alternatively be referred to as being present in its metallic form or as being a “free metal”. Thus, the metal substitute electrodes of the present invention may sometimes be referred to as “metal-free” wherein the term “metal-free” is expressly meant to embrace a material free of metal in its chemically uncombined form. Free metals typically have a form of metallic bonding that may be thought of as a type of chemical bonding that results from a sea of valence electrons which are free to move in an electronic conduction band throughout the metal lattice. While metal substitutes may contain metal constituents they are “non-metallic” on several bases. They are not pure free-metals nor are they alloys of free-metals. When metals are present in their metallic form, the electronic conduction band tends to provide, among other metallic properties, a high electrical conductivity as well as a high reflectivity for optical radiation.
A typical prior art photovoltaic device configuration is the organic bilayer cell. In the bilayer cell, charge separation predominantly occurs at the organic heterojunction. The built-in potential is determined by the HOMO-LUMO gap energy difference between the two materials contacting to form the heterojunction. The HOMO-LUMO energy levels for such a heterojunction are illustrated schematically in FIG. 1 where 101 represents an anode, 102 represents an HTL layer, 103 represents an ETL layer and 104 represents a cathode. The HOMO-LUMO gap offset between the HTL and ETL produce an electric field around the HTL/ETL interface.
Herein, the term “cathode” is used in the following manner. In a non-stacked PV device or a single unit of a stacked PV device under ambient irradiation and connected with a resistive load and with no externally applied voltage, e.g., a solar cell, electrons move to the cathode from the adjacent photoconducting material. Similarly, the term “anode” is used herein such that in a solar cell under illumination, holes move to the anode from the adjacent photoconducting material, which is equivalent to electrons moving in the opposite manner. It will be noted that as the terms are used herein anodes and cathodes may be electrodes or charge transfer layers.
Organic PV devices typically have relatively low quantum yield (the ratio of photons absorbed to carrier pairs generated, or electromagnetic radiation to electricity conversion efficiency), being on the order of 1% or less. This is, in part, thought to be due to the second order nature of the intrinsic photoconductive process, that is, carrier generation requires exciton generation, diffusion and ionization, as described above. In order to increase these yields, materials and device configurations are desirable which can enhance the quantum yield and, therefore, the power conversion efficiency.
Thompson et al. in U.S. patent application Ser. No. 09/311,126 for “Very High Efficiency Organic Light Emitting Devices Based on Electrophosphorescence”, now abandoned, have described the use of an exciton blocking layer to confine excitons to the emission layer in an organic light emitting device (OLED) in order to increase the device efficiency. In the context of the present invention, an EBL is characterized by its ability to prevent the diffusion of excitons from an adjacent organic layer into or across the EBL.
“Ultrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniques”, Chemical Reviews, Vol. 97, No. 6, 1997 (hereinafter Forrest, Chem. Rev. 1997) and Arbour, C, Armstrong, N. R., Brina, R., Collins, G., Danziger, J.-P., Lee, P., Nebesny, K. W., Pankow, J., Waite, S., “Surface Chemistries and Photoelectrochemistries of Thin Film Molecular Semiconductor Materials”, Molecular Crystals and Liquid Crystals, 1990, 183, 307, (hereinafter Arbour et al.), disclose that alternating thin multilayer stacks of similar type photoconductors could be used to enhance photogenerated carrier collection efficiency over that using a single layer structure. Further, these sources describe multiple quantum well (MQW) structures in which quantum size effects occur when the layer thicknesses become comparable to the exciton dimensions.